Thirteen people were hospitalized for heat-related illnesses at a Chicago post office Saturday morning, officials confirmed.

About 10 ambulances were called to the post office located at W Ohio and N Dearborn Streets around 8 a.m., according to the Chicago Fire Department. Thirteen people were taken to area hospitals for heat exhaustion, heat stress, and dehydration, authorities said. Another 23 refused medical treatment on the scene.

Employees said the air conditioning in the building was not working, and it was at least 100 degrees inside. 135 employees were working at the facility at the time, according to a statement from the U.S. Postal Service.

The building was evacuated and dozens of postal workers were seen standing outside in the street, on the sidewalk and inside a cooling bus following the incident.

One postal worker said he saw two people vomiting, and a supervisor who had passed out taken away in an ambulance.

Employees were allowed to reenter the building around 9 a.m. to collect their belongings in the event that they would be sent home. Around 10 a.m., postal workers were told to stay on the clock and head to the main post office at Harrison and Canal Streets in the South Loop to continue working.

If employees wanted to go home, a spokesperson said they could fill out a slip and leave. The post office will remain closed through Monday, according to the U.S.P.S.

"We regret any inconvenience this suspension may cause for our customers," acting Postmaster Tangela Bush said in a statement. "Our first priority is to ensure our employees have a safe working environment. We will resume our Fort Dearborn operations at the earliest appropriate opportunity."